The present invention relates to a method of testing the heat insulation action of bodies, especially of heat insulation products.
Heat insulation products, especially evacuated heat insulation bodies, are ideal insulating materials for refrigerating devices. They are formed of a gas-tight casing which contains e.g. a precipitated silica (such as FK 500 LS of Degussa AG) as the filler. The entire heat insulation body is evacuated to a defined internal pressure in order to raise the insulating action. The precipitated silica contained as filler in order to assure a good insulating action over a long time period exhibits a defined water content (EP-B 0,396,696). Such heat insulation bodies are produced, for example, by a process of:
a) preparing a finely distributed, powdery or fibrous material exhibiting a water absorption capacity of 4 to 50% by weight (at 23.degree. C. and 85% relative moisture) and optionally drying it under conditions sufficient for the expelling of the surface water, PA1 b) optionally pressing the powdery or fibrous material, PA1 c) placing the optionally dried and optionally pressed powdery or fibrous material in a casing which comprises an opening for evacuation and exhibits a water-vapor permeability of 0.1 to 0.5 g/(m.sup.2 .cndot.d) (at 23.degree. C. and 85% relative moisture) and gas permeabilities of 0.1 to 0.5 cm.sup.3 /(m.sup.2 .cndot.d.cndot.bar) (at 23.degree. C.), PA1 d) evacuating the casing and PA1 e) closing the evacuation opening of the casing with retention of the vacuum in the interior of the casing (EP-B 0,396,961).
The large-scale production of a rather large number of these heat insulation formed bodies has the problem of a economical and non-destructive quality control which can furnish reliable information about the quality of the heat insulation of the individual heat insulation body produced in the manufacturing operation.
It is known that the heat insulation action of evacuated heat insulation bodies can be determined by placing the heat insulation body into an evacuated chamber and investigating it there. The known method has the disadvantage that it is too troublesome and expensive for large-scale industrial use (D. Buettner et al. High Temp.--High Pressures, 15 (2), 233-40 (1983)).
Tests of heat insulation bodies show the influence of the internal pressure and of the relative moisture on the thermal conductivity coefficient; .lambda.. The thermal conductivity coefficient of a material is determined in a state of stationary heat flow after more than 12 hours dwell time and at a temperature gradient of 20.degree. C. to -20.degree. C. across the heat flow. Typical measuring times must be approximately 2 to 3 hours below stationary temperature conditions (constant heat flow in the heat insulation formed body) for heat insulation formed bodies. This amount of time is too great for a quality control program parallel to large-scale manufacture.